


Shy Heat

by SoloShikigami



Category: Undertale (Video Game)
Genre: Alternate Universe - Underswap (Undertale), Fontcest, M/M, Mating Cycles/In Heat, Multiple Orgasms, Overstimulation, Sibling Incest, Underswap Papyrus (Undertale), Underswap Sans (Undertale)
Language: English
Status: Completed
Published: 2018-10-17
Updated: 2018-10-17
Packaged: 2019-08-03 11:20:19
Rating: Explicit
Warnings: No Archive Warnings Apply
Chapters: 1
Words: 3,701
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/16325228
Author URL: https://archiveofourown.org/users/SoloShikigami/pseuds/SoloShikigami
Summary: Papyrus tries to hide his heat from Sans, but it doesn't work. It never works. Not that The Magnificent Sans ever minds helping out his dear brother.





	Shy Heat

**Author's Note:**

> A birthday gift for Butters!

****

            Sans frowned slightly when he turned from the stove to find his brother squirming in his seat at the breakfast table.

            “Is something the matter, Papy?” Sans asked as he used the spatula to place a couple of tacos on his plate. “You seem restless. Which is odd, since all you seem to ever do is rest.”

            “Heh, well, _rest_ assured, bro, I’m all right,” Papyrus replied with his usual lazy grin.

            Sans gave him a side-eyed glance that clearly said that he wasn’t buying it.

            “I’m fine, Sans, really,” Papyrus said reassuringly, picking up the taco and biting into it.

            Sans raised an eye-ridge; while he knew Papyrus enjoyed his tacos, he almost never saw him actually eating one.

            “Okay,” Sans said slowly. “If you are certain. I have training with Alphys later this morning, and I would appreciate it if you gathered the laundry while I’m gone.”

            “Sure thing, bro.”

            “I mean it, Papyrus, that also includes the sock in the living room.”

            “I thought you told me to keep it there.”

            “I DID NOT AND YOU KNOW IT!”

            “Also, you ought to be careful, I think you had some G’s in your pocket last time.”

            “What? What does that have to do with anything right now?!”

            “You know it’s against the law to launder money.”

            “OH, FOR PETE’S SAKE!”

            Sans half expected Papyrus to continue a line of puns and back jokes, but after he snickered to himself at the last one, he went oddly silent, finishing his tacos, and quickly leaving the room after putting his plate in the sink and dropping a quick kiss to the back of Sans’ skull.

            Papyrus’ behavior wasn’t the only odd thing to happen that morning, either. While the Underground was mostly impervious to change of weather, every now and then, if a strong enough storm or heat wave hit Mt. Ebot, the Underground would be affected.

            Today, it was a blizzard. Sans was alerted of this when he felt an unusual burst of cold air and heard Papyrus in the living room.

            “Fuck!” he shouted as he slammed the front door shut.

            “Language!” Sans scolded, standing in the kitchen entrance. “What has gotten into you?”

            Sans was taken aback when the response was a snarl. As quickly as Papyrus had turned angry, he switched back to normal just as fast. It seemed to have thrown him off as well, if one was to judge by how dazed he looked.

            “Sans, I, I’m so sorry,” Papyrus murmured, shaking his head in an attempt to clear it. “There’s a blizzard, we’re not going anywhere today. I, I think I’m just really, really tired. I’m, uh, going to go take a nap.”

            “Okay,” Sans said slowly, giving his brother a worried look before turning back to the kitchen to finish cleaning.

            Papyrus sighed and went up the stairs, his mind hazy, his limbs feeling shaky and weak. He knew his heat was due and his body was already starting to act up. He was hoping to go spend some time in his lab to keep his mind off of how odd he felt, but the blizzard had snowed them in and he didn’t have the reserves to teleport when he was in this state.

            When he got to the top of the stairs, his eyes lay on the door to his brother’s room, covered in caution tape and various signs. He could smell Sans’ scent through the wood and paper, and it soothed him. Without thinking, he walked inside, shutting the door quietly, and for a moment he just stood and breathed. He took a deep breath, then calm washed over him, but the tingling warmth in his bones only grew.

            “This is so annoying,” he murmured, dragging his feet a little as he went over to the blue and grey bed. He dropped down on it with a sigh, one leg hanging off the edge, and with little preamble, his hand slid under the waistband of his cargo shorts. His magic had started to form, and he was all for hurrying it along.

            “Just rub one out real quick, Sans won’t even know,” he murmured.

~ * ~ * ~ * ~ * ~ * ~ * ~ * ~ * ~ * ~

            Down in the kitchen, Sans was humming to himself as he finished wiping down the last bit of counter. He put his hand in his hips and observed the spotless kitchen. With a sigh of satisfaction, he dropped the washcloth in the laundry bin and went upstairs to check on his brother.

            He had watched Papyrus go upstairs from just around the corner to the kitchen, and he saw him going into his room (ignoring all of his signs! How rude), and it was confirming what he was certain of at breakfast.

            Papyrus was going into heat.

            It wasn’t full-blown just yet, this was more of a pre-heat, and would probably even settle a bit after some alone time, but it would come back as a full heat in a day or two.

            Sans, however, felt a shifting in his own magic the other day and was sure his cycle was coming up soon as well, so the timing wasn’t bad, all things considering.

            Papyrus has always been a shy monster; as an adult he had hidden his shyness well under a friendly disposition and his horrible puns, just a few steps above the quiet child who preferred to find someplace to hide and read astronomy and joke books. When he and Sans confessed their more than brotherly feelings for one another, Sans discovered another aspect of his shyness.

            Papyrus enjoyed sex, but very, very rarely initiated, usually didn’t ask for help when he was in heat, and when Sans finally got an answer from him about it, it was because Papyrus was embarrassed by his own reactions and was worried of what Sans would think if he ever seemed too eager or sounded weird.

            Of course, it was all preposterous to Sans, he thought Papyrus was adorable and he did his best to reassure him and make Papyrus feel as good as possible. Sans had no issues taking the lead, even seemingly forcing Papyrus into intimacy, but they had a safe word if he overstepped or read him wrong.

            Sans waited outside of his room for a moment, listening to Papyrus as he no doubt was touching himself, trying to muffle his sounds even when he was alone. With a smile and a shake of his head, Sans quietly opened the door, relishing and memorizing the scene he had now of Papyrus, his face pressed into Sans’ pillow, his hips thrusting up into his hand, and the head of his orange cock slipping in and out of it.

            _He’s so perfect and sexy,_ Sans thought.

            “My, my, dear brother,” Sans said softly, but still making Papyrus jump and stop his self-pleasuring. “You are truly the most stubborn monster I have ever come across.”

            “S-Sans!” Papyrus stammered.

            “You know that all you need to do is ask and I would do just about anything for you,” Sans continued, sauntering slowly over to the bed.

            Papyrus continued to babble, seemingly trying to get out both an apology and explanation at the same time, mumbling something about leaving the room and trying to get up, but with a small wave of his hand, Sans used his magic to pin Papyrus back to the mattress. He only struggled a little against the special attack, looking at Sans pleadingly, but said nothing. The small skeleton smiled warmly at him, brushing his cheekbone with the back of his hand.

            “My poor brother,” he murmured before climbing onto the bed and moving over the tall, prone form. “I know how you are, Papy, but you worry me so when you do this.”

            Papyrus glanced down where their bodies were just barely touching, Sans’ pelvis hovering just above his cock, which was bobbing and twitching. He thrust his hips up ever so slightly, almost unconsciously, wanting the contact but unable to ask for it.

            Sans smirked upon reading the look in Papyrus’ eyes, grunting a little as he ground down on the hardened member. Papyrus cried out, gripping Sans’ shoulders.

            “Come on, Papy, you know if you want something, you have to ask for it,” Sans said. “And you know I’m willing to do anything for you, I love you so much.”

            As Sans spoke, he continued to rub against Papyrus, who whimpered softly and wriggled beneath him. Sans bent down and captured Papyrus’ mouth in a deep kiss, continuing to press their bodies together, thrusting into him now, his own growing passion becoming more and more evident.

            “Come on, brother,” Sans prompted him again.

            “S-S-Sans,” Papyrus panted. “Sans, pl-puh-please, d-don’t, nnnng!”

            Sans pressed down particularly hard, dismissing the blue magic, and gripping Papyrus wrists, pinning them on either side of Papyrus’ head. He bent down and nuzzled his jaw.

            “What’s our safe word, brother?” Sans asked.

            “Reset,” Papyrus said clearly.

            “Good boy,” Sans kissed him again, his hands releasing the other’s wrists, sliding his hands down his arms, settling on his chest for a moment before pushing himself off. “Go back to what you were doing.”

            “Huh? Wha?” Papyrus sat up, leaning on his elbows.

            “Touch yourself again. I’m going to watch you.”

            Papyrus seemed to swallow hard as he lay back again, one slightly trembling hand moving back down to his erection, which was painfully hard now. He made a small hissing sound as he gripped it and went back to slowly stroking himself. It didn’t take him too long to fall back into the rhythm he had when Sans walked in.

            “Slow down,” Sans said after a few minutes.

            Papyrus’ hand slowed to obey.

            “Look at me.”

            Papyrus looked up, his hand still moving as his eyes locked on his brother’s. Sans slowly pulled his t-shirt up over his head, folding it carefully before dropping it on the floor. He shifted onto his knees and shimmied his shorts down just enough to pull out his own cock, rubbing his palm across the head a few times before gripping the shaft.

            “I love watching you touch yourself,” Sans murmured, his hand moving at the same pace as Papyrus’. “The look on your face as you bring yourself closer to your peak, so, so very sexy, Papyrus.”

            Papyrus shifted and looked away, blushing. Sans noticed his hands moved to the bedsheets.

            “Oh? Done already?” Sans asked.

            “Mmm,” Papyrus hummed, seeming to be unsure how to answer.

            “Mweh, heh, no, you’re not done, not by a long shot.”

            Papyrus glanced back at Sans, then his eyes widened as Sans moved to lay on his front between his legs.

            “S-Sans,” Papyrus murmured.

            “If you want it, you have to ask,” Sans said, his words whispered across the magical flesh.

            Papyrus gasped, then moaned as Sans’ tongue licked at his teeth, but he was still so close to his dick that he could feel the resonation of magic.

            “What do you want, Papyrus?”

            “I-if you want, I’d like it if you, uhm,” Papyrus looked away, the blush taking over his face completely as he mumbled, “please, suck my cock.”

            “What was that, Papyrus?”

            “Sans, please.”

            “Please what? Speak clearly, now, brother.”

            “Please suck my cock, Sans,” Papyrus said quickly, his eyes squeezed shut.

            “Of course.”

            Papyrus grunted as Sans slowly took the orange member into his mouth, his tongue curving around it. Sans hummed around him, his hands placed on his femurs to keep them apart, his thumbs rubbing gentle, soothing circles against the bone.

            “Oh, Sans, that feels so good,” Papyrus said softly, one hand caressing the top of Sans’ skull.

            “Mm, good,” Sans murmured as he pulled off for a moment. “It’s going to get better.”

            Sans took all of Papyrus in at once, his teeth scraping along the magic flesh and making Papyrus cry out. Sans bobbed his head faster, enjoying the flavor of his brother and the feel of his dick across his tongue.

            “Ooooh, Sans, yes, so good, yes, yes,” Papyrus chanted.

            Sans held Papyrus down, feeling his cock twitching and knowing that he was close. Papyrus began to whine as his orgasm approached, his breath coming out in pants, his back arching.

            “Sans, Sans, I’m, I’m gonna, g-gonna-!”

            Sans didn’t let up, in fact, one hand trailed down to Papyrus’ entrance and started to swirl his finger around it, prodding it gently but not penetrating him just yet. Papyrus’ body tensed, and he pressed himself back into the mattress, one hand slapping over his mouth as he let out a cry, and Sans swallowed it all.

            Papyrus let out a sigh of satisfaction, relaxing, his breathing slowing as Sans pulled off slowly.

            “Thank you, Sans, that felt- Ah!”

            He didn’t see Sans putting a finger in his mouth, using a bit of Papyrus’ own release to wet his finger before he began to gently push it into Papyrus’ entrance.

            “I’m not done with you, yet, brother,” Sans said. “I want to see you and hear you, and I will do what it takes to get my way.”

            Papyrus moaned as Sans twisted his finger inside of him.

            “Saaaans,” Papyrus whined.

            “What’s that, Papy? You’re ready for more?”

            “N-no, I- Mmph!”

            Sans pressed a second finger into him. Papyrus squirmed and whimpered again.

            “That’s a good boy, let me hear you,” Sans encouraged, gently beginning to scissor his fingers.

            “Sans, please,” Papyrus begged with a whine.

            “You’re enjoying yourself?” Sans asked, pressing against Papyrus’ walls, feeling out for the spots he knew he liked.

            “Y-yes,” Papyrus murmured.

            Sans smiled and continued, pressing a third finger into Papyrus and speeding up the pace.

            Papyrus’ head was spinning, feeling himself building up quickly to another orgasm. He tried to calm himself and not give into it so easily, but Sans knew him too well, knew how to touch him, what to say, and he was helpless against all of it.

            “Cum for me again, Papyrus,” Sans ordered.

            “S-Sans, n-no, I c-ca-can’t,” Papyrus tried to resist.

            “You can, and you will.”

            Papyrus knew it was inevitable, especially when Sans reached a little deeper, pressing against something inside of him that made him see stars and he cried out louder than he intended, Sans grabbing his cock and pumping him again, coaxing another peak out of him. He was still trembling from the high when he felt the smooth slide of another cock against his own. He glanced down to see Sans rubbing his dick against his, using his hand to smear his second release over them both.

            “I’m going to enter you, now, Papyrus,” Sans said. “Would you like that?”

            Even though he was already exhausted, even though it was against his better judgement as his entire body felt like it was on fire and electrified…

            “I haven’t ever wanted you to fuck me so badly, ever,” Papyrus said.

            “Language,” Sans scolded before entering him, slowly and steadily.

            Papyrus’ eyes squeezed shut again, small cries escaping his mouth with pants as he was penetrated by the thick cock he adored. Both he and Sans groaned as their pelvises met, fully joined, Sans squeezed tight by Papyrus’ inner walls, Papyrus feeling full and a burning stretch deep inside of him.

            “Stars, it feels so good to be inside of you,” Sans moaned.

            Papyrus clawed at the bedsheets weakly, too tired and wracked with pleasure at the same time to do much more. Thankfully, Sans knew him well, and began to move, still going slowly, but firm and steady.

            “Oh, Papy, you’re so beautiful,” Sans breathed. “Watching you falling apart like this is one of my favorite things. That’s why I get so annoyed when you don’t ask for help; it’s a bit selfish of me because I just want to see you like this, but I also love making you feel good.”

            Papyrus went to speak, but Sans suddenly slammed into him hard, forcing out a shocked cry instead.

            “Mweh, heh, heh, although, it is kind of fun when you resist me, because I know it’s an act. You love me, and I love you, you want me as badly as I want you, and getting you to make those lovely sounds, that pretty blush, any monster should count their lucky stars to be with you, but you’re mine, all mine.”

            Tears were stinging Papyrus’ eye sockets, between the praise and the cock continuing to thrust deeply into him, he knew he wasn’t going to last.

            “Sans, please,” Papyrus pleaded.

            “Going to cum again already? My, my, you have been quite pent up, hm?” Sans said, reaching down to slowly stroke his cock again. “By all means, brother.”

            Papyrus tried to fight it, but just like before, it was inevitable, his body shaking as it emptied again, a low moan was the only sound that could escape Papyrus now as the tears finally began falling.

            “So good,” Sans hummed. “But I think you have one more in you. After all, I’m not quite done yet, either.”

            Papyrus couldn’t protest, he couldn’t even move as Sans (without pulling out, surprisingly), carefully rolled him over onto his front, lifting his hips so his knees were propping him up, albeit a tad shakily, and his arms were pulled behind his back, his wrists captured in one of Sans’ hands.

            “How is it, Papy? Can you be a good boy for me just a little longer?” Sans asked.

            “Yes,” Papyrus whimpered.

            Later, the tall skeleton would question his sanity; even with the onset of heat, he couldn’t believe he was pushing himself this far.

            “Excellent.”

            Sans pressed down on his wrists and lower back, one hand on his hip pulling him as he thrust in, sharp and fast this time, making the bed creak as it shook under the sudden, border-line brutal assault. Papyrus cried out, the sudden pace being too much on his already overstimulated body. He felt many parts of him going numb, but it was compensated by the fire burning inside of him as he was fucked into the mattress. Sans grunted, nearly growling, chasing his own peak now, though still being sensitive to his brother’s needs by making sure he hit the spots he knew would make Papyrus see stars,

            “Papyrus, I’m going to cum soon,” Sans moaned. “Would you like that? Would you like to feel me cum inside of you? Fill you with my essence?”

            “Sans, please, please, I need it, please,” Papyrus begged breathily.

            That was all it took. With a few more thrusts, each growing gentler than the last, Sans cried out as he finally reached his own orgasm, his cock twitching deep inside Papyrus, filing his magic with his own. Papyrus was trembling underneath him, but otherwise went very still, the only sound coming out were small, soft whimpers that faded into silence. Sans carefully pulled out, a thrill in his soul making him shiver as he watched a string of pale blue cum connecting the head of his cock to Papyrus entrance, more of his magic dribbling down the curve of orange flesh. He couldn’t help swiping a finger up through the cleft, drawing a spasm from Papyrus before he went completely limp.

            “Are you all right, Papy?” Sans asked.

            Papyrus didn’t answer. Sans bent over to look at his face, and while he looked happy and satisfied, it was evident that he had passed out.

            “Oh, my, I must have pushed you a little too far,” Sans said with a small chuckle, kissing his brother’s forehead.

~ * ~ * ~ * ~ * ~ * ~ * ~ * ~ * ~ * ~

            Papyrus groaned as he awoke, his head spinning, everything in his pelvis aching, and the annoying buzz of heat still coating his bones like an unwanted bodysuit.

            “Hello there, brother. Here, drink.”

            A cup was pressed to Papyrus’ mouth, the cool water soothing on his throat and lifting the fuzzy veil that surrounded him ever so slightly.

            “Thank you,” he murmured after he was finished.

            Sans chuckled, settling back down into the bed, making sure the blanket was tucked over them both before wrapping his arms around Papyrus again.

            “Are you all right?” Sans asked.

            “Yeah. You mad at me?”

            “Annoyed that you tried to keep your heat from me, but no, not really. That was a lot of fun and it felt really good. Did it feel good for you, too?”

            “Oh, hell yeah.”

            Papyrus turned and moved so he could kiss Sans. Sans returned the kiss, deepening it a little, and the two just lingered there for a moment, their eyes meeting in a show of love and affection that didn’t need words.

            “Sans, I really am sorry, for what it’s worth.”

            “For what, Papy?”

            “That I’m like this. That I don’t ask for sex when I want it, and I even try to hide it when I need it, just… I don’t know, I guess I’m just weird.”

            “And I absolutely adore you anyway, Papyrus.”

            “God knows why.”

            “I know why, we have been down this road before. I will always love you, brother, you know that. This issue that you have regarding intimacy we have talked about, and I was sincere earlier when I said that part of me likes it,” Sans blushed and turned his face away a little. “Just knowing I’m the only one who can do this with you is, well, kind of hot.”

            Papyrus chuckled. “You’re kinda hot.”

            He turned Sans’ face back to him to kiss him again. The kisses turned more playful, as Papyrus began kissing Sans all over his face and down his neck, making Sans giggle. Papyrus finally had his fill of kissing and nuzzled Sans’ cheekbone.

            “Thank you, Sans, for being here for me, even when I’m being stupid.”

            “It’s a pleasure, brother, I love you so much,” Sans said, kissing his temple. “Now rest up, I am certain your heat is going to flair again later.”

            Papyrus blushed. “I’m sure it’ll be okay.

            With a slight roll of his eyes, Sans rolled over on top of Papyrus, smirking down at him.

            “Maybe I should pound you into the mattress again, then?” he purred.

            Papyrus’ shy switch turned on again as he blushed, but a small smirk also said that he didn’t really mind the idea in the least.


End file.
